I was able to get my immediate need on this working by changing my auto attendant transfer from a blind transfer to a monitored transfer (using an AVST CX-E voice mail here), so now the forward destination sees the VM port CPN instead of the calling number of the inbound leg.
The specific use case is callers call in, go to an auto attendant to select from 2 options, each of which is just a 5320 forwarded to an on-call person's cell phone. They need the CPN to stay consistent, so they know it's on on-call call.
But I will have to try your suggestion, Ralph. I'm curious on what happens when "Replace External CLID" is disabled on the inbound side but enabled on the outbound. I wonder which one wins, the documentation is vague about this setting.